Well boreholes are typically drilled in earth formations to produce fluids from one or more of the penetrated formations. The fluids include water, and hydrocarbons such as oil and gas. Well boreholes are also drilled in earth formations to dispose waste fluids in selected formations penetrated by the borehole.
The boreholes are typically lined with tubulars commonly referred to as casing. Casing is typically steel, although other metals and composites such as fiberglass can be used. The outer surface of the casing and the borehole wall form an annulus, which initially contains formation or drilling liquids. These liquids within the casing-borehole annulus are typically displaced with a grouting material such as cement. The casing and grouting material forming a casing-borehole sheath perform several functions. One function is to provide mechanical support for the borehole and thereby prevent the borehole from collapsing. Another function is to provide hydraulic isolation between formations penetrated by the borehole. The casing can also be used for other functions such as means for conveying borehole valves, packers, pumps, monitoring equipment and the like.
The casing-borehole annulus has traditionally been filed with heavy slurry cements. These cement slurries are pumped down the casing, flow upward into the casing-borehole annulus, and subsequently hardened or “set” thereby providing the desired formation support and isolation. In recent years, lighter weight foam cements have been used to cement casing. These light weight cements provide functions of the regular cement with the advantage of not invading the oil and gas producing soft formations Foam cements typically contain glass beads, ceramic spheres filled with air, or nitrogen bubbles. If the grouting material such as conventional or foam cement does not fill the casing-borehole annulus or is not properly bonded to the casing, hydraulic isolation is compromised. Cement evaluation systems use downhole “tools” typically measure acoustic impedance and other parameters associated with the borehole environs. Foam cement and well fluids typically have similar acoustic impedances. Distinguishing foam cement from liquid or gas well fluid is difficult using only acoustic impedance measurements from cement evaluation tools, including ultrasonic radial scanning type tools. This lack of impedance contrast often results in an indication of voids within a foam cement sheath behind casing, or that casing is “free” when it may be well bonded to foam cement.
In view of the brief discussion above, it is apparent that a measure of cement distribution behind the casing can be important from economic, operation and safety aspects. A direct measure of acoustic impedance can be used to determine the distribution of some types of grouting material in the casing-borehole annulus. Apparatus and methods for measuring acoustic impedance behind casing are disclosed in U.S. Patent Application Publication No. US 2006/0067162 A1, which is herein entered into this disclosure by reference. The difference between the acoustic impedance of convention cement and typical borehole fluids is significant. An azimuthal measure of impedance of the acoustic material in the casing-borehole annulus can therefore be used to “map” the distribution of conventional cement behind casing. Foam cement has a lower density than conventional cement and, consequently, has a lower the acoustic impedance. More specifically, the acoustic impedance of foam cement is essentially the same as typical drilling and formation fluids found in the casing-borehole annulus prior to cementing. A simple map of the acoustic impedance of material in the casing-borehole annulus can not, therefore, be used to map a distribution of foam cement behind casing.